Manhole assemblies are typically comprised of a manhole base, an intermediate or riser section and a top section normally designed to receive the manhole cover. The base section is comprised of a substantially flat base portion and a cylindrical shaped sidewall extending upwardly therefrom and integral therewith. Openings are arranged in the sidewall, each being adapted to receive the end of a pipe for selectively introducing a liquid flow into the invert or removing a liquid flow therefrom. Manhole assemblies are provided whenever a change in slope or angular orientation is encountered from one pipe run to the next. The openings receiving said pipes are arranged in accordance with the pipe runs connected thereto, the invert extending between the side-wall openings may, for example, define a straight line, right angle configuration, or a Y-configuration (in the case of a base member having three openings designed for merging two incoming pipe runs and feeding the combined flow therefrom to a single outgoing pipe run). It is extremely advantageous to maintain a smooth flow through the invert of the base member, thence turbulence resulting from misalignment of the invert relative to the incoming and outgoing pipes significantly increases the development of odious and toxic gases as a result of such turbulent conditions. In addition, a smooth fluid flow also serves to maximize flow rate through the manhole base.
Heretofore, manhole bases have typically been formed in two stages, the manhole base absent the invert being formed at the factory and the invert being formed at the job site after positioning the manhole base in the ground, usually five (5) to fifteen (15) feet below surface. Usually at least one or more workmen descend into the manhole base and set up the channel forming assemblies. The casting material, typically concrete, is also transported to the job site and dropped into the base member from ground level through the manhole assembly and into the bottom of the manhole base, dropping a distance of the order of 15 feet or more before reaching the floor of the manhole base. The workmen encounter cramped working conditions within the manhole assembly and are constrained to stand upon the channel forming apparatus during the time that the casting material is being poured, and while the casting material is setting. The workmen must also support themselves upon the channel forming apparatus in order to form the sloping surfaces in the interior of the manhole base adjacent to the invert being formed. The nature of the method steps necessary for forming an invert in accordance with the above-mentioned conventional technique in the manhole assembly base member is such that the operation is tedious, complex and time-consuming and also fails to provide accurate alignment between the invert and the sidewall openings to assure smooth flow through the manhole base and to maximize the flow rate through the manhole base.